Referring to the state of the art, FIG. 1a shows the principle of a universal planetary rolling as suggested in 1962 by Tselikov. In this system, a small square cross-sectional billet 1 is subjected to the joint action of two sets of work rolls. The first set is integral with two rotating carriers 2 and 2' which are synchronously driven rotationally about parallel axes 3 and 3'. The rotating carrier 2 carries a set of work rolls 4a, 4b, . . . 4n, distributed equiangularly along the periphery, and the rotating carrier 2' carries an identical set of work rolls 4a', 4b', . . . 4n' arranged to oppose the first set of work rolls, the axes of all work rolls being parallel with axes 3 and 3' of the rotating carriers. The second set of work rolls is integral with the two other rotating carriers 5 and 5', which are identical with the former and angularly offset through 90.degree. about the axis of billet 1-1'. The rotating carriers which are ahead and behind the plane of the figure are shown, for the sake of clarity, in thin dashed lines. Work roll 6 and 6', borne by these rotating carriers, are the same in number and layout as work rolls 4 and 4', but so arranged that they act on the two other sides of billet 1-1' during the interval that separates the respective actions of two successive pairs of work rolls 2 and 2' belonging to the carriers, for instance, the pairs 4a, 4a' and 4b and 4b'. In this way, the cross section of billet 1 is gradually reduced through the action of opposing pairs of work rolls alternately on a pair of opposite sides, and then on the other. FIG. 1b shows the shape taken by the rolled bar in the part where it is in contact with the work rolls running through their respective paths. This part is called the roll gap, this term also being applicable to the portion of the bar located in this part and extending from the point where the deformation begins up to the sectional plane P' at which the cross-section of the billet has the desired dimensions and which is accordingly called "exit plane".
However, in this embodiment, the generatrix of the work rolls is straight so that the rolling operation is carried out, widening taking place freely: the ridging effect caused by the sharp corners A (FIG. 1c) due to the substantial superficial widening this type of deformation entails, results in significant cooling of the corners of the billet, formation of angular cracks and surface defects detrimental to the quality of the finished product.
To obviate this drawback, it has tentatively been suggested (European patent application issued under No. 0000290 in the name of Hill Engineering Company Limited) to use work rolls whose profile includes a shoulder with a rounded angle which rolls at every pass the relevent corner of the product. However, to achieve this end, the axes of the work rolls must be at a 45.degree. angle to that of the rotating carrier which bears them; this involves a complication of the transmission system, jeopardizes the reliability of the operation and leads to substantially increased lateral space requirements for the machine.